Correct.
I cannot think of a workaround here.
If you do not have too many students you can probably pick one question in the quiz, and override student grades for that question manually, one-by-one.
Or, if you have some multiple choice question that is worth 2 points, you can edit that question and make all answers there worth 100%. Then click Regrade. Everyone who answered wrong would be awarded 2 extra points, and then you will still need to manually override grades for those students who answered that question right.
I think this would confuse the students though.
I know I cannot manually change the quiz marks. But I can enter an assignment called Quiz completion and use that to put in a zero if they have not done the quiz. When they do the quiz, I delete the zero. You could use this to add 2 to every mark (i.e. make the 12 a 14 on the 'assignment') and then set the weight to zero for that quiz. It might work.